1. Field of the Invention
The present invention relates to the preparation of lactams by ring-closing hydrolysis of the corresponding aminonitriles.
2. Description of the Prior Art
Aliphatic lactams, in particular epsilon-caprolactam are basic starting materials for the production of polyamides (e.g., polyamide 6 from caprolactam).
One known technique for preparing these lactams entails a ring-closing hydrolysis of the corresponding aminonitriles, more particularly of unbranched aliphatic aminonitriles, by passing them in vapor phase, with water, over a solid catalyst.
Thus, U.S. Pat. No. 2,357,484 describes a process for the vapor phase preparation of lactam, by passing a mixture of water and of aminonitrile over a dehydration catalyst such as activated alumina, silica gel and titanium dioxide or borophosphoric acid.
U.S. Pat. No. 4,628,085 discusses a process for the preparation of lactams in the vapor phase, by contacting an aliphatic or aromatic aminonitrile and water with a catalyst based on silica in the form of spherical particles which have a BET specific surface area greater than 250 m.sup.2 /g and a mean pore diameter smaller than 20 nm, and generally in the presence of hydrogen and ammonia.
In general, the catalysts employed in the processes of the prior art enable good selectivity for the lactam to be attained. On the other hand, it is often the case that the catalysts are rapidly deactivated and this presents a serious drawback when carrying out the process on an industrial scale.
Moreover, the process described in U.S. Pat. No. 4,628,085 employs a very complex reaction mixture requiring, when the reaction is complete, separation and recycling operations which greatly complicate the process.